Eight Perfect Murders by Peter Swanson

Malcolm Kershaw owns the Old Devils Bookstore in Boston. One day, an FBI agent named Gwen Mulvey interviews him about a blog post he wrote several years ago listing the eight mystery novels that featured what he considered to be the perfect murder. Apparently, there have been some recent crimes where the manners in which the victims were killed are similar to The ABC Murders by Agatha Christie and Double Indemnity by James M. Cain, which were two of the books Malcolm featured. Agent Mulvey wonders if Malcolm has any connection to the murders. Malcolm, himself, appears to be an unreliable narrator, with secrets of his own, so the reader wonders--does he?